Description ID of the account that is the parent of this contact. We recommend that you only update up to 50 contacts simultaneously when changing the accounts on contacts enabled for a Customer Portal or partner portal. We also recommend that you make this update during times outside of your organization's business hours.
It is an ID of the account that is the parent of this contact.
Hope this helps.
Please mark this as solved if it's resolved.
Regards,
Nagendra.
1) https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_objects_contact.htm
AccountId
Type
reference
Properties
Create, Filter, Group, Nillable, Sort, Update
Description
ID of the account that is the parent of this contact.
We recommend that you only update up to 50 contacts simultaneously when changing the accounts on contacts enabled for a Customer Portal or partner portal. We also recommend that you make this update during times outside of your organization's business hours.
Let us know if this will help you